NEW YORK — Michael Avenatti, the pugnacious attorney best known for representing porn actress Stormy Daniels in lawsuits against President Trump, was arrested Monday on charges that included trying to shake down Nike for as much as $25 million by threatening the company with bad publicity.

Avenatti, who was also accused of embezzling a client’s money to pay his own expenses, was charged with extortion and bank and wire fraud in separate cases in New York and California. He was arrested in New York just minutes after tweeting that he planned to hold a news conference Tuesday to ‘‘disclose a major high school/college basketball scandal perpetrated by @Nike that we have uncovered.’’

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‘‘When lawyers use their law licenses as weapons, as a guise to extort payments for themselves, they are no longer acting as attorneys. They are acting as criminals,’’ said Geoffrey S. Berman, the US attorney in New York.

California investigators had been building a case against Avenatti for more than a year, but prosecutors in New York said their investigation began only last week and was completed in days.

In the California case, Avenatti allegedly misused a client’s money to pay his debts and those of his coffee business and law firm. Federal prosecutors said he also defrauded a Mississippi bank by using phony tax returns to obtain millions of dollars in loans.

The allegations ‘‘paint an ugly picture of lawless conduct and greed,’’ said Nick Hanna, the US attorney in Los Angeles. Avenatti describes himself on Twitter as an attorney and advocate, but the accusations describe ‘‘a corrupt lawyer who instead fights for his own selfish interests.’’

The arrest was a sharp reversal of fortune for the 48-year-old lawyer, who, less than a year ago, emerged as a leading figure in the anti-Trump movement, with relentless cable news appearances, a hard-punching style and a knack for obtaining information about others’ wrongdoing.

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Donald Trump Jr. gloated over the arrest on Twitter.

‘‘Good news for my friend @MichaelAvenatti, if you plead fast enough, you might just get to share a cell with Michael Cohen!’’ he wrote, referring to the former Trump lawyer set to go to prison next month for crimes including orchestrating hush-money payments to Daniels.

Avenatti allegedly threatened to hold a news conference last week on the eve of Nike’s quarterly earnings call and the start of the NCAA tournament to announce allegations of misconduct by Nike employees. The attorney and a co-conspirator demanded to be paid $15 million to $25 million and an additional $1.5 million for an Avenatti client to remain silent, the complaint said.

Two people familiar with the investigation confirmed that the unidentified co-conspirator was Mark Geragos, a Los Angeles criminal defense lawyer known for his work with celebrities. The people spoke on condition of anonymity because the information was not made public by prosecutors.

Geragos, a CNN contributor, has a client list that has included Michael Jackson, Winona Ryder, Scott Peterson and most recently Jussie Smollett, the actor accused of fabricating a racist, anti-gay attack in Chicago. Geragos did not respond to messages seeking comment. Within hours, CNN cut ties with him.

The Avenatti client is a coach of an amateur athletic union men’s basketball program in California, according to the papers. The AAU program coached by the client was sponsored by Nike for $72,000 annually, the complaint said.

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Shortly before the charges came to light, Avenatti tweeted that he planned to hold another news conference regarding Nike on Tuesday. Less than 45 minutes later, prosecutors announced the extortion case and his arrest.

While lawyers sometimes make demands to seek out-of-court settlements, they cannot threaten to go public with damaging information to get something of value or gain leverage in a civil dispute, attorney Neama Rahmani said.

‘‘The Department of Justice historically has been very cautious when charging attorneys, so they likely have evidence that Avenatti seriously crossed this line,’’ said Rahmani, a former federal prosecutor.

Nike officials told investigators that Avenatti claimed to know of rules violations by an amateur basketball team sponsored by Nike. Executives immediately reported the threats to federal authorities.

The company ‘‘firmly believes in ethical and fair play, both in business and sports, and will continue to assist the prosecutors,’’ Nike said in a statement.

Avenatti was in custody and did not respond to an email requesting comment or phone calls and text messages from The Associated Press.

He rose to national prominence by representing Daniels, whose real name is Stephanie Clifford, in a lawsuit to break a confidentiality agreement to speak about her alleged affair with Trump. He also made headlines in recent weeks for representing two women who accused R&B star R. Kelly of sexual abuse, and he briefly explored the idea of a presidential bid last year.

Daniels said she was ‘‘saddened but not shocked’’ by the arrest. She issued a statement Monday on Twitter saying she fired Avenatti a month ago after ‘‘discovering that he had dealt with me extremely dishonestly.’’ She said she would not elaborate.

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While Avenatti’s lawsuit effectively tore up the gag order that threatened financial penalties if Daniels spoke about the case, a defamation lawsuit filed on her behalf against Trump backfired, and a court ordered her to pay the president’s $293,000 in legal fees.

Avenatti himself has been dogged with tax and financial troubles in recent years.

A US bankruptcy court ordered his former firm to pay $10 million to a lawyer who claimed it had misstated its profits. The Justice Department also alleged Avenatti made ‘‘misrepresentations’’ in the bankruptcy case and said his former firm still owed more than $440,000 in unpaid federal taxes, which Avenatti dismissed as ‘‘part of a smear campaign.’’ He said he did not personally owe any of the money.

The bank fraud case involved $4 million in loans he got from Peoples Bank in Biloxi, which prosecutors said he obtained by filing fraudulent tax returns claiming $14 million in income over three years. However, he never filed tax returns those years, nor paid the $2.8 million he reported on the forms. In fact, he still owed more than $850,000 to the IRS at the time for previous income.

Mark Pearson, the assistant agent in charge of the IRS office in LA, said Avenatti’s crimes supported a lavish lifestyle that included a $200,000-a-month lifestyle, a car racing venture and pricey homes in the wealthy Orange County communities of Newport Beach and Laguna Beach.